A material handling machine such as a front-end loader used in agricultural or earth-moving industries typically includes a mobile-powered frame with a hydraulically operated lift arm structure connected thereto. The lift arm structure or boom assembly usually includes a pair of load-lifting arms which are each connected to the frame. Each lift arm is typically provided with its own driver for raising and lowering the respective lift arm. A loader bucket or other suitable implement is connected to a forward or distal end of each lift arm.
Most lift arm structures are also provided with a cross-piece transversely extending between the lift arms. The primary purpose of the cross-piece is to add strength and rigidity to the lift arm structure. The ends of the cross-piece are typically welded to both side surfaces of each lift arm.
During loader operation, relatively high stresses are repeatedly imparted to the lift arm structure. Such stresses include torsional as well as bending stresses. These stresses can result from any one or a series of different loader operations. As an example, a bending stress may be imparted to the lift arm structure when the loader is driven into a pile of material that is to be loaded into the bucket. As may be appreciated, the drivers connected to each of the lift arms tend to impart torsional stresses to the lift arm structure when opposite corners of the bucket have unequal vertical loads applied thereto.
It is not desirable to design the bucket to absorb the twisting and bending stresses applied to the lift arm structure. It is known, however, to provide the cross-piece with a generally circular and tubular configuration to absorb the stresses between the lift arms. The diameter of such a tube is generally restricted to the vertical side surface or width of the lift arms. To add rigidity to the lift arm structure, the wall thickness of the tubular cross-piece is maximized. Although increasing the wall thickness of the cross-piece tube will make it strong, the increase in size likewise reduces its flexibility and causes the relatively high stresses imparted to the lift arm structure to be transferred to the ends of the cross-piece rather than absorbed therebetween.
Most lift arm structures are fabricated as weldments and are, therefore, only as strong as the welds which secure the pieces together. As is appreciated, welds are extremely brittle. Their brittleness causes them to crack or fracture easily especially when relatively high stresses are repeatedly applied thereto. The high stresses imparted to the ends of a known cross-piece have been known to cause the welds securing the cross-piece to the lift arms to crack and fracture. Failure of the welds imperils the strength, rigidity and general operativeness of the lift arm structure.